Second Black Hole for the Milky Way

Just half a light-year from the supermassive black hole in the center of the Milky Way, there may be another, much smaller black hole, embedded in a doomed cluster of stars. The discovery supports a popular scenario for the formation of medium-sized black holes, and it may explain the unexpected existence of young, massive stars in the core of the Milky Way.

There are two main types of black holes. Stellar black holes are the remains of exploded stars. They are just a few times heftier than the sun. On the other end of the scale are supermassive black holes, which lurk in the cores of galaxies and can weigh several billion times the mass of the sun. In recent years, astronomers using x-ray satellites have found evidence of medium-sized black holes, a few thousand times the sun's mass. They are thought to form in dense stellar clusters from chance collisions of massive stars.

One such stellar cluster, known as IRS 13E, lies close to the Milky Way's supermassive black hole. Jean-Pierre Maillard of the Astrophysical Institute in Paris and his colleagues studied IRS 13E using large telescopes at Mauna Kea, Hawaii, and in Chile. They discovered at least six bright stars that appear to be traveling through space together, suggesting that the stars are held together by the gravity of a massive central object. In a paper published in Astronomy & Astrophysics, Maillard and his colleagues estimate the mass of this putative black hole at 1300 times the mass of the sun.

According to theoretical astrophysicist Simon Portegies Zwart of the University of Amsterdam, the Netherlands, the find is "a fantastic confirmation" of a theory he and Steve McMillan of Drexel University in Philadelphia, Pennsylvania, put forward 2 years ago to explain the existence of young stars in the center of the Milky Way. These stars presented a puzzle because they are close to the central supermassive black hole, where the clouds of gas and dust needed to build stars would be disrupted by the hole's powerful tidal forces. However, based on computer simulations, Portegies Zwart and McMillan proposed that dense stellar clusters that formed farther out in a galaxy could spiral into the core and survive the pull of the supermassive black hole--if they were held together by a smaller black hole.

Richard Mushotzky of NASA's Goddard Space Flight Center in Greenbelt, Maryland, agrees that this is "potentially a fantastic result." However, he says, it would be much more clear-cut if IRS 13E was spewing x-rays, like the medium-sized black hole candidates in other galaxies.